Chemoselective Transfer Hydrogenation of Flavoring Unsaturated Carbonyl Compounds over Zr and Hf-based Metal–Organic Frameworks

Full text of DOI:10.1002/bkcs.12226

Communication
DOI: 10.1002/bkcs.12226
BULLETIN OF THE
A. H. Valekar et al.
KOREAN CHEMICAL SOCIETY
Chemoselective Transfer Hydrogenation of Flavoring Unsaturated
Carbonyl Compounds over Zr and Hf-based Metal–Organic
Frameworks
†
†,‡
†,§,
*
Anil H. Valekar, Kyung-Ryul Oh, and Young Kyu Hwang
†Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology, Daejeon 34114,
Korea
‡
Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea
§
Department of Advanced Materials and Chemical Engineering, University of Science and Technology,
Daejeon 34113, Korea. *E-mail: ykhwang@krict.re.kr
Received October 30, 2020, Accepted January 4, 2021, Published online February 1, 2021
Keywords: Metal–organic frameworks, Metal-node modification, Catalytic transfer hydrogenation,
Unsaturated carbonyl compounds
The chemoselective hydrogenation of a carbonyl group in
multiunsaturated aldehydes and ketones is a difficult task
reaction occurs through the procedure called Meerwein–
Ponndorf–Verley reduction (MPV). Typically mild reac-
1
14
because hydrogenation of C C bonds is favored thermody-
namically over C O bonds. Allylic alcohols are mostly
tion conditions are applied for MPV reduction to minimize
the risk of reducing other functional groups or unsaturated
C C bonds.
2
used in the fine chemical industry for the production of
3–5
pharmaceuticals, flavors, perfumes, and cosmetics. Many
unsaturated carbonyls, which possess aroma and flavoring
properties, can be produced using extraction from plant
sources. However, the supply and the quality of traditional
In the last few years, zirconium-based catalysts have been
15–18
attracting interest in this regard.
Hydrous zirconia proved
to be a good catalyst for the catalytic transfer hydrogenation
(CTH) reaction of cinnamaldehyde and gave a yield of 78%
6
15
natural-flavor and fragrance chemicals are limited. Cur-
cinnamyl alcohol.
Moreover, transformation could be
19
rently, around 90% of newly introduced fragrance ingredi-
achieved under flow conditions. Isomorphous substituted
“Zr” in the framework of zeolite Beta (Zr-Beta) with only a
small amount of Zr, achieved 96% cinnamyl alcohol yield
from cinnamaldehyde. In recent years, MOFs and MOF-
derived materials have been used successfully as heteroge-
neous catalysts in several chemical transformations.
Consequently, Zr-containing MOFs have recently been tested
for CTH of various carbonyl compounds.
7
ents are synthetic aroma chemicals. Benzaldehyde,
cinnamaldehyde, citral, and carvone are widely used as ref-
erence unsaturated aldehydes. Cinnarizine, the cardiovascu-
20
8
lar drug, is derived from cinnamyl alcohol, whereas
21–24
geraniol and nerol (derived from citral) are useful for the
9
production of perfumes, food flavors, and insecticides.
25–27
Vanillin is the most widely used flavor compound in the
Our research
world and lignin, a major component of lignocellulosic bio-
group has also contributed to the successful transformation of
various biomass-derived carbonyl compounds using series of
10
mass, could be utilized for the production of vanillin.
1
1
Homogeneous Ru-phosphine-diamine catalyst and Rh
Zr-based functional and metal node-modified MOFs under
12
28,29
(I) complexes
showed excellent performance for
mild reaction conditions.
Acid–base properties originated
chemoselective reduction of these carbonyl compounds
from metal nodes regarded as active sites for the CTH reac-
tion using Zr-MOFs.
using molecular H . However, the requirement for an extra-
2
base for its activation, difficulties in recovery, and loss in
catalytic activity in recycling convinced many researchers
to use heterogeneous catalysts. Typically, the reductions of
these carbonyls were performed under high-pressure molec-
Herein, we report the results of our investigation on the
effect of metal sites in MOF-808, a highly active catalyst for
the CTH reaction. Hf-MOF-808 was tested along with Zr-
MOF-808 for the chemoselective reduction of α, β-unsaturated
carbonyl compounds. Five different flavoring compounds were
selected considering their huge potential in the flavor and
aroma industries.
ular H with supported metal nanoparticle catalysts such as
2
1
3
Ni, Pt, Pd, Ru, and Rh. The process is complex because
olefins are often preferentially reduced to form major prod-
ucts (saturated aldehydes or ketones). Therefore, tremen-
dous efforts have been made to overcome this problem
over the past decades. The alternative method used for
chemoselective hydrogenation of an α, β-unsaturated alde-
hyde or ketone is catalytic transfer hydrogenation. The
The synthesized Zr- and Hf-MOF-808 was initially char-
acterized using powder XRD. All structural peaks in the
synthesized MOFs were well matched with their simulated
30
patterns (Figure 1(a)). This indicated their highly crystal-
line nature. Thermal gravimetric measurements were
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performed to evaluate the thermal stability and degradation
patterns in the synthesized Zr- and Hf-MOF-808 catalysts.
Figure 1(b) shows that three different weight-loss patterns
for both the MOFs can be observed. These correspond to
Acid–base sites in the Zr-MOFs were already proved as
an important aspect of the CTH reactions. These sites in
Zr-MOFs are generated from the metal node, which pos-
sesses Zr O Zr bonding and was well discussed in our
28,29
previous work.
Therefore, acid and base sites were
ꢀ
physisorbed water (around 100 C), removal of formate,
measured using NH3 and CO2 temperature-programmed
desorption (TPD), respectively (Supporting information
Figure S1). Furthermore, Corma et al. have also reported
the superior Lewis acidity of Hf-MOF-808 relative to that
of Zr-MOF-808 using fourier-transform infrared (FTIR)
μ -OH, and solvent molecules (DMF) in the region of
3
ꢀ
2
50–350 C, and structural degradation that happened due
to the elimination of an organic linker (BTC) in the
ꢀ
4
00–550 C region.
32
Porous properties were determined using N2 phy-
with CD CN as a probe molecule, whereas Farha et al.
3
sisorption isotherms measured at 77 K (Figure 1(c)). Both
evaluated the acidity of Zr and Hf-based MOFs using
33
these MOFs showed multistep N adsorption features rep-
potentiometric acid–base titration.
Zirconium and
2
resenting microporous as well as near mesoporous proper-
ties. The porous properties of these MOFs, along with the
metal composition determined from ICP are summarized in
the Supporting Information Table S1. The brunauer-
emmett-teller surface area of Zr-MOF-808 is better
hafnium-based MOF-808 catalysts can selectively convert
these unsaturated carbonyl compounds into their respective
alcohols via transfer hydrogenation that utilizes secondary
alcohol (isopropanol) as a hydrogen source (Scheme 1). No
subsequent reduction of C C has been observed to produce
saturated alcohols confirmed from GC analysis; therefore,
the reaction proceeds with high chemoselectivity toward
C O groups to produce allylic alcohols.
2
2
(1407 m /g) than that of Hf-MOF-808 (1047 m /g); how-
ever, a similar trend has been observed previously in the lit-
31
erature. The pore-size distribution calculated from the
density-functional theory method (Figure 1(d)) showed that
Zr-MOF-808 possesses bigger pores (inner pore diameter
Table 1 summarizes the results obtained from the CTH
reaction of five substrate molecules over Zr- and Hf-MOF-
ꢀ
1
2.7 Å and 18.6 Å for tetrahedral and adamantine cages,
respectively) than those of Hf-MOF-808 (11.8 Å and
5.9 Å). This implies that these MOFs are very porous
materials.
808 with different time intervals at 82 C to compare the cat-
alytic activity of the MOFs tested here. The CTH reaction is
highly dependent on the electronic structure of the substrate
molecules, as is evident from the different reaction rates for
1
(a)
(b) 100
Hf-MOF-808
Zr-MOF-808
Simulated MOF-808
Zr-MOF-808
Hf-MOF-808
80
60
40
20
0
5
10
15
20
25
30
100 200 300 400 500 600 700
O
2
ꢀ (degree)
Temperature ( C)
(c)
(d)
8
6
4
2
00
00
00
00
0
Zr-MOF-808
Hf-MOF-808
Zr-MOF-808
Hf-MOF-808
6
4
2
0
0
.0
0.2
0.4
0.6
0.8
1.0
10
15
20
25
30
Relative pressure (P/P0)
Pore Width (Å)
2
Figure 1. (a) Powder XRD patterns, (b) TG curves, (c) N adsorption–desorption isotherms at 77 K, and (d) pore-size distributions of Zr-
MOF-808 and Hf-MOF-808.
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be attributed to a steric hindrance to forming a six-membered
cyclic ring involving a carbonyl group, IPA and metal node
from MOF (which is a transition state that transfers a hydride
ion from IPA to a carbonyl group) on the catalyst surface.
Comparing the catalytic activity of both Zr- and Hf-MOF-
8
08 toward all five substrates, it could be determined that
Hf-MOf-808 has higher activity than Zr-MOF-808. At the
maximum conversion, the two catalysts showed not much
difference in conversions or in selectivity; however, their real
reaction rate could be judged by comparing them at lower
conversion levels. A similar trend was observed in all the
CTH reactions carried out here. Furthermore, under the cur-
rent applied reaction conditions, equilibrium was reached for
the carvone after reaching ~70% conversion. Although
a slight augmentation in conversion was observed using Hf-
MOF-808 by continuing the reaction for 12 h (Supporting
Information Figure S2). The CTH reaction proceeds via
MPV reduction, which is a reverse equilibrium reaction
called Oppenauer oxidation.
Scheme 1. Chemoselective hydrogenation of α, β-unsaturated car-
bonyl compounds using the CTH method over Zr- and Hf-based
MOF-808 catalysts.
different molecules. Citral and cinnamaldehyde, which do
not have aromatic α-carbon can readily and quantitatively
convert into respective alcohols in 2 h of reaction time. On
the other hand, both benzaldehyde and vanillin have aro-
matic α-carbon. Among these two compounds, vanillin took
a longer reaction time (5 h) to produce a nearly quantitative
yield of vanillyl alcohol due to the presence of electron
donating groups on the aromatic ring. The electronic effect
of these substituted groups could lead to an increase in elec-
tron density around carbonyl carbons and, subsequently,
hydride ion attack from the alcohols hindered. This ham-
pered the reaction rate. Moreover, the hydrogenation of
ketone (carvone) is slower than that of aldehydes. It could
The differences in catalytic activity of Hf and Zr catalysts
observed here may be caused by a greater electron affinity
for Hf, causing stronger binding to reactants and translating
to a lower apparent activation energy. Koehle et al. studied
the MPV reduction reaction of furfural over Sn, Zr, and Hf
34
containing siliceous zeolite Beta. They found that activa-
tion energy for Hf-Beta (49.6 kJ/mol) was much smaller than
that of Sn- and Zr-Beta zeolites (60.7 kJ/mol and
Table 1. CTH of carbonyl flavoring compounds over Zr/Hf-MOF-808.
Catalyst
Zr/Hf-MOF-808)
Time
(h)
Conversion
(%)
Selectivity
(%)
TOF
(h )
Substrate
Product
-1
(
1
2
1
2
1
2
78/88
99/100
81/85
99/99
83/88
99/99
97/98
97/98
8.0/9.1
5.1/5.2
8.2/8.8
5.0/5.1
8.8/9.3
5.2/5.2
Zr/Hf
Zr/Hf
Zr/Hf
96/98
96/98
100/100
100/100
2
5
57/68
92/97
100/100
100/100
3.0/3.6
2.0/2.1
Zr/Hf
Zr/Hf
1
8
10/19
68/73
100/98
1.1/2.0
0.9/1.0
100/100
ꢀ
Reaction conditions: Substrate—2.6 mmol, IPA—20 mL, catalyst—9.5 mol % (based on metal), and temperature—82 C (reflux). TOF = (moles
of substrate)/(moles of Zr/Hf from ICP analysis × time).
Bull. Korean Chem. Soc. 2021, Vol. 42, 467–470
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6
0.4 kJ/mol, respectively). Accordingly, a similar trend was
11. T. Ohkuma, M. Koizumi, H. Doucet, T. Pham, M. Kozawa,
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performance over Sn- and Zr-Beta catalysts. Despite the sim-
ilar ionic radii of Zr and Hf (0.73 Å and 0.72 Å), the better
performance of Hf-MOF-808 could be related to the slightly
stronger Lewis acidic character of Hf (relative to that of Zr);
however, a detailed investigation is essential to support this
claim. For this, FTIR analysis using reliable probe molecules
would be highly desirable for calculating the exact amount
and strength of Lewis acid, and the base sites.
In summary, both Zr-MOF-808 and Hf-MOF-808 were
found to be active and chemoselective catalysts for the CTH
reaction of α, β-unsaturated carbonyl compounds. The selec-
tivity to all allylic alcohols was >97% throughout the reac-
tion. Excellent conversions were always paired with high
selectivity. The slightly better performance of Hf-MOF-808
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A.H.V. and Y.K.H. are deeply grateful to the Institutional
Research Program of KRICT (SI2111-40) for financial sup-
port. The authors would like to thank Dr. U-H. Lee, D.-Y.
Hong, and Dr. D. W. Hwang for helpful discussions.
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Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
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